It provides the United States Air Force (USAF) with a heavy intercontinental-range strategic airlift capability, one that can carry outsized and oversized loads, including all air-certifiable cargo.

In 1961, several aircraft companies began studying heavy jet transport designs that would replace the Douglas C-133 Cargomaster and complement Lockheed C-141 Starlifters.

In addition to higher overall performance, the United States Army wanted a transport aircraft with a larger cargo bay than the C-141, whose interior was too small to carry a variety of their outsized equipment.

The criteria were finalized and an official request for proposal was issued in April 1964 for the "Heavy Logistics System" (CX-HLS) (previously CX-HLC).

After a downselect, Boeing, Douglas, and Lockheed were given one-year study contracts for the airframe, along with General Electric and Pratt & Whitney for the engines.

[11][12] Boeing lost the military contract but went on to develop the successful 747 civilian airliner with over 1,500 aircraft built when manufacturing ended in 2022 after 54 years of production.

Flight tests revealed that the aircraft exhibited a higher drag divergence Mach number than predicted by wind tunnel data.

[14] After being one of the worst-run programs, ever, in its early years, it has evolved very slowly and with great difficulty into a nearly adequate strategic airlifter that unfortunately needs in-flight refueling or a ground stop for even the most routine long-distance flights.

A passive load-reduction system, involving uprigged ailerons, was incorporated, but the maximum allowable payload was reduced from 220,000 to 190,000 lb (100,000 to 86,000 kg).

[20] The Deputy Assistant Secretary of the Air Force for Management Systems, Ernest Fitzgerald, was another person whose fostering of public accountability was unwelcome.

Lockheed delivered the first operational Galaxy to the 437th Airlift Wing, Charleston Air Force Base, South Carolina, in June 1970.

Due to higher than expected development costs, in 1970, public calls were made for the government to split the substantial losses that Lockheed was experiencing.

During static and fatigue testing, cracks were noticed in the wings of several aircraft,[18] and as a consequence, the C-5A fleet was restricted to 80% of maximum design loads.

[48] In 2014 Lockheed investigated drag reduction by plasma-heating of turbulent transonic airflow in critical points, saving overall weight by reducing fuel consumption.

[49] The C-5 is a large, high-wing cargo aircraft with a distinctive high T-tail fin (vertical) stabilizer, with four TF39 turbofan engines mounted on pylons beneath wings that are swept 25°.

[citation needed] The cargo hold of the C-5 is one foot (30 cm) longer than the entire length of the first powered flight by the Wright brothers at Kitty Hawk, North Carolina.

The rear main landing gear can be made to caster to make a smaller turning radius, and rotates 90° after takeoff before being retracted.

"Kneeling" landing gear permits lowering the aircraft when parked, thereby presenting the cargo deck at truck-bed height to facilitate loading and unloading operations.

The nose and aft cargo-bay doors open the full width and height of the cargo bay to maximize efficient loading of oversized equipment.

[citation needed] The C-5 Galaxy is capable of moving nearly every type of military combat equipment, including such bulky items as the Army armored vehicle launched bridge, at 74 short tons (67 t), from the United States to any location on the globe;[citation needed] and of accommodating up to six Boeing AH-64 Apache helicopters or five Bradley Fighting Vehicles at one time.

During the Yom Kippur War in 1973, multiple C-5s and C-141 Starlifters delivered critical supplies of ammunition, replacement weaponry and other forms of aid to Israel, the US effort was named as Operation Nickel Grass.

Operational deployment was discarded due to engineering and security difficulties, though the capability was used as a negotiating point in the Strategic Arms Limitation Talks.

[69][70][71] C-5s have routinely delivered relief aid and humanitarian supplies to areas afflicted with natural disasters or crisis; multiple flights were made over Rwanda in 1994.

[82] As of November 2013, 45 C-5As have been retired, 11 have been scrapped, parts of one (A/C 66-8306) are now a cargo load trainer at Lackland AFB, Texas, and one was sent to the Warner Robins Air Logistics Center (WR-ALC) for tear down and inspection to evaluate structural integrity and estimate the remaining life for the fleet.

[90] On 18 July 2017, C-5s based at Dover were ordered to stand down so maintenance crews could determine the cause for some nose landing gear failing.

Modifications also included adding a second inlet for ground power, which can feed any power-dependent equipment that may form part of the cargo.

[citation needed] Proposed during the 1990s Non-Developmental Airlift Aircraft (NDAA) program as an alternative to further purchases of the McDonnell Douglas C-17 Globemaster III as well as a replacement for older C-5As.

It includes fitting new General Electric F138-GE-100 (CF6-80C2) engines, pylons and auxiliary power units, and upgrades to aircraft skin and frame, landing gear, cockpit and pressurization systems.

[44][101] Each CF6 engine produces 22% more thrust (50,000 lbf or 220 kN),[102] providing a 30% shorter takeoff, a 38% higher climb rate to initial altitude, an increased cargo load and a longer range.

[106] Lockheed proposed a twin body C-5 as a Shuttle Carrier Aircraft to counter the Conroy Virtus, but the design was turned down in favor of the Boeing 747.